1. Field of the Invention
The invention relates to an optical aerial line and to a method of installation thereof.
2. Description of the Related Art
It is known to combine an optical communication system with aerial power system, by routing utility cables, such as telephone or data transmission cables, between selected locations as overhead wiring suspended from a series of utility poles.
Fiber optical cable links are the foundation of such communications systems, since they have the advantage of large capacity, high speed and long distance transmission, without showing any cross-talk. At the same time, they are not influenced by electromagnetic fields, which is very important for installations on High-Voltage (HV) lines.
The easiest way to connect power plants and control stations is to use the existing high voltage lines. The most common method for this is to install an Optical Ground Wire (OPGW), which contains optical fibers, as a substitution of an existing ground wire. Another possibility consists in incorporating optical fibers in a cable the replaces one of the standard phase conductors. This solution is called Optical Phase Conductor (OPPC). Besides these integrated solutions, additional cables can be installed on the towers. Self-supporting cables for additional installation on high-voltage lines are the MASS (Metallic Aerial Self-Supporting) and the ADSS (All-Dielectric Self-Supporting) cables. While these self-supporting cables are hung between the towers, the ADL (All-Dielectric Lashed) is a small sized cable that is attached to a messenger wire. In HV lines, either a ground wire or a phase conductor may serve as the messenger.
The article of R. Böhme, R. Girbig, G. Hög (Alcatel Kabel AG & Co, Mönchengladbach, Germany), “Fibre Optic Lashed Cables on High Voltage Lines”, International Wire & Cable Symposium Proceedings 1998, pages 642-649, presents an overview of existing techniques for attaching a light weight dielectric optical fibre cable to an existing rope, either ground wire or phase conductor. The optical cable may be lashed onto a messenger by means of a lashing binder, wrapped onto the messenger, attached to the messenger by means of clips or clamps, or attached by means of a preformed rod.
The authors of this article consider the lashed cable technique as the one that offers more installation flexibility and allows for longer spliceless cable length than any other technique. The optical cable may be lashed to the messenger with two dielectric lashing binders, the second one being added only for safety reasons.
U.S. Pat. No. 4,424,954 (Communications Systems) relates to a device to lash a fiber optic cable to a supporting strand and tackles the problem of insufficient securing of the optical cable when a prior art cable lashing machine is used with recent fiber optic cables, which are lighter and smaller than in the past. Due to the insufficient securing, the fiber optic tends to twist around the supporting strand, and such twisting damages the insulation on the fiber optic cable and thus reduces the cable's efficiency and impulse transmitting ability. The problem is eliminated by tensioning the fiber optic cable as is passes through the lashing machine and is lashed to the supporting strand, so as to maintain it coextensive relative to the supporting strand. A lashing line is then helically wrapped around the supporting strand and the optical cable.
U.S. Pat. No. 6,193,824 (Siemens) relates to a method of installing an optical cable on a conductor of a high-voltage overhead line with the aid of securing elements applied in a helical manner, wherein a safeguard against coming off in the event of a rupture of a securing element is provided. The optical cable is fitted along the electrical conductor and secured by means of a helically-wrapped adhesive tape. A particularly secure attachment results if two adhesive tapes are applied with crossover wrapping since there is a further defined bonding between the two adhesive tapes additionally at the adhesive tape crossover points.
The Applicant notes that, although the use of adhesive tape provides a good coupling of the optical cable and the electrical conductor, problems of ageing of the adhesive can arise, which can reduce in time the effectiveness and the safety of the coupling.
The Applicant has tackled the problem of improving the tightness and the safety of a lashed coupling, avoiding the use of adhesives, which are subjected to ageing, and of cable tensioning techniques, which are difficult to implement. The Applicant has in particular tackled the problem of avoiding displacements of the optical cables after installation, due for example to wind which would affect the integrity and the performances of the optical line.
The Applicant has verified that a particularly convenient arrangement of the optical/electrical lashed assembly is to dispose the optical cable parallel to the electrical conductor and underneath the electrical conductor (in a vertical-alignment geometry). In this case, possible lateral displacements of the optical cable are undesired. The Applicant has for example observed that, in very cold environmental conditions, a lateral displacement of the optical cable with respect to the position underneath the electrical conductor would lead to an increase ice formation on the assembly, and therefore to an additional weight to be supported by the structure. Moreover, this displacement of the optical cable renders the structure more subjected to oscillations when the ice layer is detached from the assembly (galloping phenomena).